My son is 15 months old and we are expecting a girl in 7-8 weeks. We have started talking about "baby sister" and we talk about baby sister's chair, car seat, room, etc. I know he doesn't understand it now, but am hoping that by talking about it it will help when she arrives. We look at books that have babies and talk about being soft with babies. Any other resources or recommendations on how to prepare him (as much as a 17 month old can be prepared) for the arrival of a sibling so close in age?
Congrats on the move to man-to-man parenting! First, I suggest you start figuring out now how to schedule some one-on-one time with him. My daughter spent the first two months after her brother arrived telling me she didn't love me anymore, which was her way of telling me she needed attention too. We started having Mommy-Daughter and Daddy-Daughter dates, and the jealousy melted away and has not returned, so long as we continue that tradition with both of them.
I'd also try to find ways he can help with his little sister when she arrives. Can he fetch a new diaper when one is required? How about tummy time; can he be in charge of hanging out with her on the mat and showing her new toys to look at? The more involved he feels in how the family works as a unit, the less alone and neglected he might feel when little sister needs attention.
It sounds like you're doing a pretty good job of preparing him for the positive aspects of having a baby sister. The negative side is easier to overlook. Primarily, he is accustomed to receiving your instant and undivided attention, which he will not be able to get as often. A lot of parents anticipate this by showering him with attention now, and there's nothing really wrong with that, except it can make the transition more of a shock for him.
If you haven't already, I would start working on helping him be more independent. If you have been feeding him, help him learn to feed himself. If you have been rocking him to sleep for 30 minutes every time, help him get used to a shorter time. If he doesn't know how to play quietly, help him start to learn now. If he never plays by himself, help him gradually get used to that. If he insists on one parent always doing a certain task with him, help him get used to the other parent helping out with that. That way he won't feel like suddenly his parents never take care of him anymore. Start making it a positive thing to act like a "big boy" now, so it won't feel like a negative thing to him after the baby comes.